Twelve patients with advanced stages of hematologic malignancies (acute non-lymphocytic leukemia (ANLL) - 2, acute lymphocytic leukemia -2, Hodgkin's disease - 1) and solid tumors (melanoma -2, adenocarcinoma, colon (ACC) - 3, metastatic adenocarcinoma, unknown primary (MAUP) -1, oat cell carcinoma, lung -1) received 75 g/sq. m/day of Thymidine (TdR) given as a continuous infusion IV for 5 days. Hematologic toxicity is summarized as follows: leukopenia (57 percent), thrombocytopenia (29 percent), and anemia (52 percent) with average nadirs of 2.5/cu. mm (range 0.9 - 3.8), 97.5/cu. mm (range 28 - 136) and 27.3 volume percent (range 22.6 - 29.3) on median days 7, 5.8, and 4.6 respectively. Non-hematologic toxicities are gastrointestinal: nausea, vomiting and anorexia (91 percent), diarrhea (73 percent), and indigestion (22 percent), and central nervous system: somnolence (82 percent), severe headaches (73 percent), visual illusions (56 percent), and memory impairment (22 percent). Alopecia occurred only in patients who had received more than 5 courses. No renal or hepatic toxicity or allergic manifestation were observed. Thymine crystals were noted in the patients urine after refrigeration. Tumor regression (PR) occurred in 1 patient with melanoma. Three out of four patients with acute leukemia had decrease in peripheral WBC counts and blasts but no bone marrow response. One patient with ANLL also had shrinkage or hepatolosplenomegaly. Three patients with ACC and 1 with MAUP had stable disease. High pressure liquid chromatography was used for measuring plasma concentrations of TdR and T-ymine. Millimolar concentrations were reached within 12 hours and maintained during the entire period of treatment. The plasma half-life of TdR was approximately 100 minutes. Forty to sixty-seven percent TdR and 13 - 18 percent thymine were excreted in the urine. One-third of the plasma TdR concentration was measurable in the cerebro-spinal fluid. Further trials are ongoing to evaluate the efficacy of TdR given at 75 g/ sq. m/day as a continuous infusion.